collector: limit pme context note to user-defd fns

rustc adds notes to errors which happen post-monomorphization to
provide the user with helpful context (as these errors may rely on the
specific instantiations). To prevent this note being added where it is
not appropriate, the node is checked to originate outwith the current
crate. However, when polymorphization is enabled, this can result in
some errors (produced by `optimized_mir`) to occur earlier in
compilation than they normally would, during the collection of shims.
Some shims have ids that originate in the standard library, but these
should not receive the PME note, so instances for compiler-generated
functions no longer receive this note.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_middle/src/mir/mono.rs

@@ -47,6 +47,14 @@ pub enum MonoItem<'tcx> {
 }
 
 impl<'tcx> MonoItem<'tcx> {
+    /// Returns `true` if the mono item is user-defined (i.e. not compiler-generated, like shims).
+    pub fn is_user_defined(&self) -> bool {
+        match *self {
+            MonoItem::Fn(instance) => matches!(instance.def, InstanceDef::Item(..)),
+            MonoItem::Static(..) | MonoItem::GlobalAsm(..) => true,
+        }
+    }
+
     pub fn size_estimate(&self, tcx: TyCtxt<'tcx>) -> usize {
         match *self {
             MonoItem::Fn(instance) => {

compiler/rustc_monomorphize/src/collector.rs

@@ -450,7 +450,9 @@ fn collect_items_rec<'tcx>(
     // involving a dependency, and the lack of context is confusing) in this MVP, we focus on
     // diagnostics on edges crossing a crate boundary: the collected mono items which are not
     // defined in the local crate.
-    if tcx.sess.diagnostic().err_count() > error_count && starting_point.node.krate() != LOCAL_CRATE
+    if tcx.sess.diagnostic().err_count() > error_count
+        && starting_point.node.krate() != LOCAL_CRATE
+        && starting_point.node.is_user_defined()
     {
         let formatted_item = with_no_trimmed_paths(|| starting_point.node.to_string());
         tcx.sess.span_note_without_error(